1. Field of the Invention
The present invention relates to source follower circuits and more particularly to source follower circuits with lower output impedances.
2. Background Information
Source follower circuits are ubiquitous in circuitry especially analog circuitry. They are often found as interface circuits where the source follower: presents a high impedance (to not load down) to a sending circuit; presents a low impedance (to not diminish the signal) to a receiving circuit; and demonstrates a gain of one.
Source followers extends a line of follower circuits from cathode followers (tubes) to emitter followers (bipolar transistors). Basically all follower circuits present a gain of one, a high impedance to the sending circuit and a low impedance to a receiving circuit.
FIG. 1 illustrates a known source follower circuit with an added MOSFET M2.
The source of M1 is driven by a current source I1, and the drain of M1 and the gate of M2 are driven from a current sink I2. The drain current of M2 will be I1 minus I2, and I1 is set higher than I2.
Assuming the current sources have very high output impedances the equation for the output impedance for FIG. 1 is:Rout=1/(gm1)(gm2)(ro1).  Eq. 1
Here gm1 is the transconductance of M1, gm2 is the transconductance of M2 and ro1 is the output impedance of M1.
The drain voltage of M1 is set by the gate-source voltage of M2. When the IN signal in FIG. 1 approaches ground or below ground, the drain-source voltage of M1 may approach the saturation voltage of M1 driving M1 out of its saturation region and causing non-linearities in the OUT signal.